1. Field of the Invention
The present invention is generally related to pressure vessels and in particular to the replacement of nozzles in pressure vessels.
2. General Background
Pressure vessel components such as pressurizers in a nuclear steam system have nozzles that experience material failure due to stress corrosion cracking. This is a result of the susceptibility of the material, high stresses, and operation in a caustic environment. When a nozzle is found to contain cracks or there is leakage around the nozzle, it must be repaired or replaced.
A presently used replacement method that applicants are aware of consists of cutting off the existing nozzle at the pressure vessel shell and drilling out the remaining portion of the nozzle within the vessel wall. A j-groove weld preparation is then ground into the remaining original structural weld while insuring that a certain minimum amount of buttering material cover remains in the bottom of the groove. This is necessary to avoid welding too near the base materials which would then require weld heat treatment after welding in accordance with ASME code requirements. This is avoided because weld heat treatment of a portion of a pressurizer in this situation is impractical. The weld preparation surface and one-half inch beyond the weld preparation on adjacent surfaces then must be dye penetrant examined. A small void that is inherent to the weld process used to deposit the original structural weld requires an excessive amount of grinding to obtain cleared surfaces that are suitable for welding. The grinding process must be performed by personnel working inside the pressure vessel in a highly radioactive and uncomfortable environment. The process results in program delays, radiation exposure to personnel, and a risk to the successful completion of the work.
Patents that applicants are aware of that are directed to the replacement of sleeves or nozzles include the following.
U.S. Pat. No. 4,440,339 discloses a method for repairing the housing of a control drive mechanism. The housing is cut at a point between the weld joint with the sleeve and the inner surface of the reactor vessel. The lower portion of the housing is removed and a new housing is inserted into the sleeve and welded to the remaining housing portion.
U.S. Pat. No. 4,615,477 discloses a method for replacing tubes in a header or drum without removing the entire tube. The old tube is cut off level with the header and a recess is machined into the header to create a seat for a new tube. The remaining old tube is beveled and welded to the header at the bevel and a new tube is inserted into the seat and welded to the exterior of the header.
U.S. Pat. Nos. 5,094,801 and 5,091,140 disclose an apparatus and method for replacing a heater sleeve in a nuclear reactor coolant system pressurizer. The original heater and nozzle are removed and the bore enlarged. An outer sleeve is installed in the bore on the same center as the original sleeve. An inner sleeve is then installed in the outer sleeve to maintain the original heater alignment.
The problems associated with the void area that remains after a nozzle has been removed is not adequately addressed by the known art with regard to the time required to complete the repair and personnel exposure.